The present invention relates to a light source unit that is usable in an optical apparatus such as a projector and uses a light emitting element such as a semiconductor laser and an optical fiber, and to a projector.
For example, in an image display projector such as DLP (registered trademark) projector and a liquid crystal projector and in a photomask exposure apparatus, a high luminance discharge lamp (HID lamp) such as a xenon lamp and an ultra-high pressure mercury lamp has been used so far.
As an example, a principle of a projector is described with reference to FIG. 4 (see Japanese Unexamined Patent Application Publication No. 2004-252112, etc.). FIG. 4 is a diagram explaining a part of one kind of existing projector.
Light from a light source (SjA) formed of a high luminance discharge lamp or the like enters an incident end (PmiA) of a homogenizing means (FmA) with the help of focusing means (illustration thereof is omitted) formed of a concave reflector or lens, and entering light is output from an exit end (PmoA).
Here, as the homogenizing means (FmA), for example, a light guide may be used. The light guide is also referred to as a rod integrator, a light tunnel, or the like, and is configured of a prism formed of a light transmissive material such as glass and a resin. When the light guide is used as the homogenizing means (FmA), light entering the incident end (PmiA) is totally reflected repeatedly by side surfaces of the homogenizing means (FmA) and propagates in the homogenizing means (FmA), in accordance with the principle same as that of the optical fiber. Accordingly, even if distribution of light entering the incident end (PmiA) has unevenness, the homogenizing means (FmA) configured of a light guide functions to sufficiently uniformize illuminance on the exit end (PmoA).
Note that, in addition to the light guide configured of a prism formed of a light transmissive material such as glass and a resin described above, there is a light guide that is a hollow square tube and whose inner surface is configured of a reflector. The light guide of this type performs the same function as that of the light guide configured of a prism, by propagating light while allowing the light to be reflected repeatedly by the inner surface.
Further, in the projector of FIG. 4, an illumination lens (Ej1A) is disposed such that a square image of the exit end (PmoA) by the light output from the exit end (PmoA) is formed on a two-dimensional light intensity modulator (DmjA). Therefore, the two-dimensional light intensity modulator (DmjA) is illuminated with the light output from the exit end (PmoA). Incidentally, in FIG. 4, a mirror (MjA) is disposed between the illumination lens (Ej1A) and the two-dimensional light intensity modulator (DmjA).
Then, the two-dimensional light intensity modulator (DmjA) directs the light to a direction entering an image projection lens (Ej2A) for each pixel, according to a picture signal. Alternatively, the two-dimensional light intensity modulator (DmjA) so modulates the light as to direct the light to a direction not entering the image projection lens (Ej2A) for each pixel, to display an image on a screen (Tj).
Note that the two-dimensional light intensity modulator as described above is also called light bulb, and in the case of the optical system of FIG. 4, normally, DMD (registered trademark, digital micro-mirror device) is often used as the two-dimensional light intensity modulator (DmjA).
In addition to the above-described light guide, the homogenizing means includes a fly eye integrator. A principle of a projector using the fly eye integrator as the homogenizing means is described with reference to FIG. 5, as an example (see Japanese Unexamined Patent Application Publication No. 2001-142141, etc.). FIG. 5 is a diagram explaining a part of one kind of an existing projector.
In the projector of FIG. 5, light from a light source (SjB) configured of a high luminance discharge lamp or the like enters, as substantially parallel luminous flux, an incident end (PmiB) of the homogenizing means (FmB) configured of a fly eye integrator with the help of collimator means (illustration thereof is omitted) formed of a concave reflector, lens, or the like, and entering light is output from an exit end (PmoB).
Here, the homogenizing means (FmB) is configured by combination of a front fly eye lens (F1B) on incident side, a rear fly eye lens (F2B) on exit side, and an illumination lens (Ej1B).
Each of the front fly eye lens (F1B) and the rear fly eye lens (F2B) is formed by arranging a plurality of square lenses that has the same focusing distance and the same shape vertically and horizontally.
Each of the front fly eye lenses (F1B) and the rear fly eye lens (F2B) corresponding thereto configure Kohler illumination optical system, and thus, a plurality of Kohler illumination optical systems are arranged vertically and horizontally.
Typically, Kohler illumination optical system is configured of two lenses and illuminates a target surface (a surface desired to be illuminated) uniformly. At the time of illuminating the target surface by collecting light with a front lens, the two lenses are disposed so that the front lens forms a light source image not on the target surface but on a center of a surface of a rear lens and the rear lens forms an image of a square of an outer shape of the front lens on the target surface. The action of the rear lens is to prevent phenomenon occurred in a case where the rear lens is not provided, specifically, phenomenon in which, when the light source is not a complete point light source and has a finite size, illuminance in the periphery of the square image on the target surface is dropped depending on the size. It is possible to uniform illuminance over to the periphery of the square image on the target surface by the action of the rear lens, without depending on the size of the light source.
Here, in the case of the optical system in FIG. 5, substantially parallel luminous flux basically enters the homogenizing means (FmB). Therefore, the front fly eye lens (F1B) and the rear fly eye lens (F2B) are disposed such that a distance therebetween becomes equal to the focusing distance thereof, and therefore, an image on the target surface of uniform illumination as Kohler illumination optical system is generated to the infinity. Incidentally, since the illumination lens (Ej1B) is disposed on a rear stage of the rear fly eye lens (F2B), the target surface is drawn on a focusing surface of the illumination lens (Ej1B) from the infinity.
Each of the plurality of Kohler illumination optical systems arranged vertically and horizontally is parallel to an incident optical axis (ZiB), and luminous flux enters each of the Kohler illumination optical systems substantially axisymmetrically to the center axis thereof. Therefore, output luminous flux is also axisymmetrical. Accordingly, images of the outputs of all of the Kohler illumination optical systems are formed on the same target surface on the focusing surface of the illumination lens (Ej1B) by property of the lens in which light beams entering a lens surface at the same angle are so refracted as to travel toward the same point on the focusing surface irrespective of incident positions of the respective light beams on the lens surface, namely, Fourier transform function of the lens.
As a result, illumination distributions on the respective lens surfaces of the front fly eye lenses (F1B) are all overlapped, and thus a synthesized square image whose illuminance distribution is more uniform than that in the case of one Kohler illumination optical system, is formed on the incident optical axis (ZiB). When the two-dimensional light intensity modulator (DmjB) serving as an illumination target is disposed on the position of the synthesized square image, the two-dimensional light intensity modulator (DmjB) is illuminated with light output from the exit end (PmoB). Incidentally, in the illumination, a polarization beam splitter (MjB) is disposed between the illumination lens (Ej1B) and the two-dimensional light intensity modulator (DmjB) to reflect the light toward the two-dimensional light intensity modulator (DmjB).
Then, the two-dimensional light intensity modulator (DmjB) modulates the light and reflects the modulated light such that the polarization direction of light for each pixel is rotated by 90 degrees or is not rotated, according to a picture signal. As a result, only the rotated light passes through the polarization beam splitter (MjB) and enters an image projection lens (Ej3B), thereby displaying an image on the screen (Tj).
Incidentally, in the case of the optical system in FIG. 5, typically, LCOS (registered trademark, silicon liquid crystal device) is often used as the two-dimensional light intensity modulator (DmjB). In a case of such a liquid crystal device, only a component of light in a specified polarization direction is effectively modulated. Therefore, a component of light parallel to the specified polarization direction is normally transmitted as is. However, in the optical system in FIG. 5, a polarization aligning device (PcB) that rotates polarization direction of only a component of light perpendicular to the specified polarization direction by 90 degrees and consequently allows all of light to be effectively used may be interposed, for example, on a rear stage of the rear fly eye lens (F2B). In addition, for example, a field lens (Ej2B) may be interposed immediately before the two-dimensional light intensity modulator (DmjB) so that substantially parallel light enters the two-dimensional light intensity modulator (DmjB).
Incidentally, in addition to the reflective two-dimensional light intensity modulator illustrated in FIG. 5, a transmissive liquid crystal device (LCD) in a compatible optical arrangement is also used as the two-dimensional light intensity modulator (see Japanese Unexamined Patent Application Publication No. H10-133303, etc.).
Incidentally, in a typical projector, to perform color display of an image, for example, a dynamic color filter such as a color wheel is disposed on the rear stage of the homogenizing means to illuminate the two-dimensional light intensity modulator with color sequential luminous fluxes of R (red), G (green), and B (blue), and color display is achieved time-divisionally. Alternatively, a dichroic mirror or a dichroic prism is disposed on the rear stage of the homogenizing means to illuminate the two-dimensional light intensity modulator that is provided independently for each color, with light color-separated to three primary colors of R, G, and B, and a dichroic mirror or a dichroic prism is disposed to perform color synthesis of the modulated luminous fluxes of the three primary colors of R, G, and B. However, to avoid complication, these are omitted in FIG. 4 and FIG. 5.
However, the high luminance discharge lamp disadvantageously has low conversion efficiency from supplied power to optical power, namely, large heating loss, short lifetime, or the like.
As an alternate light source overcoming these disadvantages, a solid light source such as an LED and a semiconductor laser has attracted attention in recent years.
Among them, the LED has smaller heating loss and longer life time as compared with the discharge lamp. However, light radiated from the LED does not have directivity similarly to the discharge lamp, and thus usage efficiency of light is disadvantageously low in an application using only light in a certain direction, such as the projector and an exposure apparatus.
On the other hand, the semiconductor laser has a disadvantage that speckle occurs due to high coherency, but the disadvantage is overcome by various technical improvement such as usage of a diffuser plate. Since the semiconductor laser has small heating loss and long lifetime similarly to LED and has high directivity, the semiconductor laser advantageously has high usage efficiency of light in application using only light in a certain direction, such as the projector and the exposure apparatus described above. Moreover, the semiconductor laser utilized high directivity to perform optical transmission by optical fibers with high efficiency. Therefore, it is possible to separate the installation position of the semiconductor laser from the position of a projector or the like using the light. Consequently, it is possible to enhance flexibility of device designing.
Incidentally, even in the case where the same current flows, the brightness of the semiconductor laser varies due to environment temperature variation or temperature increase by self heating, and further due to deterioration associated with increase of accumulated conduction time. Therefore, in the case where the semiconductor laser is applied to a projector, feedback control may be desirably performed in order to stabilize light quantity. To realize the stabilization of light quantity, means to measure a light quantity is necessary, and in particular, an optical sensor to measure the light quantity of each of colors R, G, and B at a light inlet of the projector, namely, at exit ends of the optical fibers, may be desirably provided.
Incidentally, the optical fiber has disadvantage of risk of breakage because the optical fiber is made of vulnerable glass such as quartz in spite of convenience.
For example, in a case where a projector having brightness of ten thousand ANSI lumen, it is necessary to transmit optical power of about 200 W by optical fibers, depending on efficiency of the optical system. Accordingly, if the optical power is transmitted by six optical fibers, it is necessary to transmit optical power of 30 W or higher per one optical fiber. Incidentally, when the number of optical fibers is increased, power per one optical fiber is decreased. However, this increases cost, and therefore, it is difficult to increase the number of optical fibers immoderately. Moreover, if the optical fiber transmitting such large power is broken, the optical power is leaked from the broken point, and is absorbed by a covering material provided for mechanically protecting the optical fibers, which may result in fire damage of the covering material. Therefore, when the optical fiber is broken, safety measures that detect the breakage of the optical fiber to turn off the semiconductor laser are necessary.
For the optical fiber, a technology to detect breakage has been developed from high power application in which such fire damage of a component may occur to low power application for communication, etc.
For example, in Japanese Unexamined Patent Application Publication No. H06-050841, there is disclosed a technology mainly for an optical fiber for communication in which light with a wavelength to be transmitted and monitor light with a wavelength different from the light with the wavelength to be transmitted are transmitted from transmission side, a filter that reflects and returns the monitor light is provided on reception side, and presence of return of the monitor light is monitored on the transmission side to detect breakage of the optical fiber.
Further, in Japanese Unexamined Patent Application Publication No. H09-269248, there is disclosed a technology mainly for an optical fiber for communication in which presence of breakage of an optical fiber is detected based on a waveform of returned light at the time when pulsed light enters the optical fiber, and a distance to a break point is calculated if the optical fiber is broken.
Moreover, in Japanese Unexamined Patent Application Publication No. H10-038751, there is disclosed a technology mainly for a high-power optical fiber for laser processing in which an optical sensor detecting stray light in a lens system is disposed near the lens system on each of incident end side and exit end side of the optical fiber, and detected light quantity of both of the optical sensors are compared to detect breakage of the optical fiber.
Further, in Japanese Unexamined Patent Application Publication No. H11-005187, there is disclosed a technology mainly for a high-power optical fiber for laser processing in which a protection tube covering the optical fiber is provided, a plurality of sensors each detecting laser light leaked from the optical fiber are arranged inside the protection tube, and thus breakage of the optical fiber is detected.
Further, in Japanese Unexamined Patent Application Publication No. H11-344417, there is mainly disclosed a technology in which light with a wavelength to be transmitted and monitor light with a wavelength different from the light with the wavelength to be transmitted are transmitted through a high-power optical fiber for laser processing from transmission side, and presence of the monitor light is monitored on reception side to detect breakage of the optical fiber.
Further, in Japanese Unexamined Patent Application Publication Nos. 2002-350694 and 2004-219244, there are disclosed technologies mainly for an optical fiber for communication or for laser processing in which a conductive covering film is provided on an outer peripheral surface of the optical fiber, and conduction state between incident end side and exit end side is monitored to detect breakage of the optical fiber.
Further, in Japanese Unexamined Patent Application Publication No. 2003-279444, there is disclosed a technology mainly for a high-power optical fiber for laser processing in which a cable including a plurality of temperature fuses connected in series is disposed along the optical fiber, and breakage of the optical fiber is detected based on melting of the temperature fuses.
Further, in Japanese Unexamined Patent Application Publication No. 2006-064399, there is disclosed a technology mainly for an optical fiber for communication in which presence of breakage of an optical fiber is detected based on a phase difference between incident modulated light and returned modulated light, and a distance to a break point is calculated if the optical fiber is broken.
Further, in Japanese Unexamined Patent Application Publication No. 2012-147860, there is disclosed a technology mainly for an optical fiber for endoscope illumination in which breakage of an optical fiber is detected based on a ratio between quantity of entered light and quantity of returned light that are measured at a light incident end of the optical fiber.